Composite ceramic/glass substrates have recently seen rapidly increased applications in the electric/electronic industries. Ceramic/glass composite substrates, by definition, comprise primarily a glass matrix with fine ceramic particulate fillers distributed therewithin. Since glass constitutes the major part of the ceramic/glass substrates, the type and composition of the glass component determine the basic properties of the substrates. On the other hand, ceramic fillers are added to change or improve the physical and/or chemical properties of the substrate. By changing the types and compositions of glass and ceramic components, as well as the proportions thereof, a ceramic/glass substrate can be tailored to suit a particular application.
In the manufacturing of multichip modulus in the electronic packaging industry, for example, the glass component used in making the ceramic/glass composite in a multilayer ceramic substrate can be selected from the family of glass materials with low dielectric constant and low thermal expansion-coefficient, such as borosilicate glass; and the ceramic fillers can be selected from a variety of ceramic materials such as fused silica, cordieritc, forsteritc, aluminum oxide etc. The proportions between the glass and the ceramic components can be properly adjusted so that the composite materials can be sintered and densifted at temperatures between 800.degree. and 1,000.degree. C. The ceramic/glass composite can be used in conjunction with conductor circuit made from low-melting point and low-resistivity metals, such as gold, silver, copper, etc, to form multilayer ceramic substrates, which exhibit low dielectric constant and with a thermal expansion coefficient close to that of silicon chips. These multilayer ceramic substrates can be made to achieve the same high packaging density, high wiring density and high reliability as silicon chips. With a proper design, ceramic/glass based multilayer substrates can bring the benefits of reducing the delay in signal transfer in the substrate as well as the noise level produced therefrom. With these advantages, ceramic/glass based multilayer substrates can be produced which meet the stringent specifications required for use in large mainframe computers as well as in super computers.
Composite ceramic/glass substrates can also be used in the manufacturing of hard memory discs for storing data in computers. One of the advantages thereof is that excellent flatness can be obtained due to the ability of softened glass material to lay flat at elevated temperatures. Fine ceramic particles can be added to the glass matrix to form surface protrusions with heights ranging from tens to hundreds of angstroms. Composite ceramic/glass substrates allow the production of high-hardness and high-flatness hard disc substrates to be therefrom with appropriate surface texture. Other advantages of using ceramic/glass composite materials include a simplified procedure in the manufacturing of hard discs, and the increased density of the discs made therefrom for storing more data therein per unit area.
Due to their low thermal expansion coefficient and excellent thermal shock resistance, some glass materials such as borosilicate glass and aluminosilicate glass can be used, in conjunction with high mechanical strength and high thermal conductivity ceramic fillers such as aluminum oxide, silicon carbide, silicon nitride, etc., in the manufacturing of hot plates, heating discs and far-infrared emitters that contain internally embedded heating circuit.
With the above-mentioned advantages, composite ceramic/glass substrates certainly have a very promising potential. However, due primarily to their inherent lack of mechanical strength, the applicability of composite ceramic/glass substrates is actually quite limited.
One of the reasons for the limited applicability of ceramic/glass based articles is that, due to its fragility, any surface defect caused by, for example, minor cracking or scratching will grow inwardly resulting in a total failure of the ceramic/glass based article. The growth of surface defect becomes more profound under a tensile stress. To avoid the occurrence of such failure, ceramic/glass and/or glass based materials are strengthened, typically by imparting a compressive stress at the surface thereof to suppress the growth of surface defects. In the presence of such compressive stress, In order to counter such compressive stress, a greater measure of tensile stress will be required in order for the surface defects to grow. The existence of such compressive stress, therefore, suppresses the growth of surface defects and improves the strength of the ceramic/glass based articles.
Several methods have been disclosed in the prior art to form the compressive stress at the surface of glass material. Generally they can be classified into five major types: ion exchange method; thermal tempering method, controlled surface crystallization method; laminating method; and surface coating method.
Ion exchange method was discussed, for example, in U.S. Pat. No. 4,726,981, in which strengthened glass articles are prepared by contacting glass bodies, at a temperature above the annealing point of the glass, with a source of Li.sup.+ ions to replace part of the Na.sup.+ ions and, if present, K.sup.+ ions in a surface layer with a corresponding amount of Li.sup.+ ions and to react the Li.sup.+ ions with Al.sub.2 O.sub.3 and SiO.sub.2 in the surface layer to form crystallites of beta-quartz solid solution nucleated by TiO.sub.2 and/or ZrO.sub.2. The content of the '981 patent is incorporated by reference. Ion exchange method has the advantage that it involves a relatively mature technology therefore has a wide range of applicability. It can be applied not only to glass materials, but crystalline materials as well. Its disadvantages are that distortion can result if article is too thin, and that salt particles often adhere to the surface thereof after the heat treatment process thus causing pollution problems. The ion exchange method also has the disadvantage that it is not applicable to articles that contain electronic parts and/or circuit.
Thermal tempering method involves a relatively simple procedure. After the glass article is formed, cold air is blown upon the hot glass article to reduce its surface temperature to that below its annealing temperature. Because the surface layer cools faster, it is subject to relatively greater volumetric contraction. However, since the inner layer is at a higher temperature and a viscous flow is still present, no tensile stress is exerted on the outer layer. The glass article is then cooled at a lower rate. Because the surface layer is at a lower temperature at the beginning of the second stage, it has a relatively small extent of volumetric contraction relative to the inner layer. This causes a compressive stress to be exerted on the surface layer. Because the temperature of the surface is lower than the annealing temperature during the second cooling stage, such compressive stress can be preserved to provide the strengthening effect. This method is typically applicable to glass or glass-rich articles with simple geometry. However, this method is relatively difficult to control and the strengthening effect disappears at elevated temperatures, thus it only has a relatively narrow range of areas in which it can be applied.
Controlled surface crystallization method has been discussed, for example, in U.S. Pat. Nos. 4,341,543 and 4,218,512, the content thereof is herein incorporated by reference. In '543, a method of making strengthened glass-ceramic articles is disclosed which comprises the steps of subjecting a glass article to vapors of SO.sub.2 and thereafter heat treating the glass article to cause crystallization in situ thereof. '512 discloses a similar method of strengthening glass-ceramic articles by creating a surface compressing layer by controlling the crystallization process to yield different crystalline forms at the surface and in the interior. Some of the major disadvantages of the controlled surface crystallization method include: difficulty in controlling the treatment process, variation in the physical and mechanical properties between different batches, and relatively poor reliability. Furthermore, the controlled surface crystallization method also suffers from the need to use a multiplicity of treatment steps and an assortment of unknown factors involved in the various steps, as well as the relatively high manufacturing cost. Also, this method can be applied only to a limited range of glass-ceramic materials and in a limited number of applications.
U.S. Pat. No. 3,649,440, whose content is incorporated by reference, teaches a method for supplemental strengthening of laminated articles of glass by forming a plurality of fused adjacent laminae by thermal tempering such as to greatly increase the impact resistance of the articles. Each lamina exhibits a state of stress opposite to that of the lamina contiguous thereto. The advantage of the laminating method is that the thickness of the strengthened layer and the compressive stress can be controlled by controlling the thickness and composition of glass laminae. However, the laminating method requires a relatively high treatment temperature (1,350.degree. C.); therefore, it cannot be used in making glass articles that contain electronic parts or screen printing circuit. Furthermore, if the glass laminae are too thin, uniform thickness cannot be obtained; thus, the laminating method cannot be applied in the manufacturing of thin strengthened glass plates.
The surface coating method involves primarily depositing on the surface of sintered articles, using a chemical vapor deposition (CVD) or sputtering method, a material having lower thermal expansion coefficient. U.S. Pat. No. 4,781,970, whose content is incorporated by reference, discussed the surface coating method, by which ceramic articles are strengthened by forming a compressive material layer of amorphous silicon dioxide or refractory metal nitride on the surface of the article to be strengthened. One of the major advantages of the surface coating method is that the thickness of the strengthened layer can be made very thin; also, this method can be used in strengthening ceramic substrates containing electronic parts and circuit. However, the surface coating method has several disadvantages such as: the thickness of the strengthened layer is limited (it cannot exceed several microns), and the strengthened article does not tolerate any surface scratching. These disadvantages severely limited the applicability of the surface coating method.
All the methods disclosed in the prior art involve melting the raw material by heating the same to a temperature above the melting point thereof, cooling the molten material to form glass plates or other shapes, then applying surface treatment to form the compressive stress and thus provide the strengthen effect.
There exist, however, other techniques which do not require the melting of the glass material to form glass or glass/ceramic articles. In these techniques, glass and/or glass/ceramic powders are used to prepare precursory substrates having a desired shape using a variety of powder forming methods such as tape casting, dry pressing, extrusion, or injection molding method, etc. The precursory substrates are then sintered at a temperature substantially below the melting temperature of glass to form densifted composite ceramic/glass articles. The ceramic/glass articles prepared from the sintering process, however, do not have adequate strength; generally the maximum strengths of sintered ceramic/glass articles are only about 150-180 MPa. Inadequate strength is a major weakness of using the sintering process to manufacture glass/ceramic articles.